Exhaust Backfire or Afterfire

[cyberborikua]

I own a 1979 Fiat Spider to which we recently changed the stock intake to a 1800 and added a 34ADF carb. It runs beautifully, but I notice an exhaust backfiring after each shift. This did not happen before the conversion. The car was set to 10 rpm in idle and seems to run well. The backfire is not too bad, but sometimes it makes a kind of a loud bang. Could it be an exhaust leak? Could it damage the exhaust system? Thanks in advance for your ideas and thoughts.

[azruss]

The backfire can be from an exhaust leak around the manifold. Since you didnt have it before the carb change, i would think that the carb is creating a lean condition when you take your foot off the gas. This suggests a jetting issue. You are doing your cat, resonator and muffler no favors with that much backfire.

[RoyBatty]

The firing out the exhaust is after each shift, and seems to me it would indicate a rich condition or late timing or maybe both on dropping the throttle.
In changing the manifold are you sure all vacuum connections are properly routed?
If there is a vacuum line to the distributor, is it connected to ported vacuum and is it secure?
Seems to me that in the early smog control systems, there used to be a device just for the purpose of preventing firing out the exhaust when you let off the throttle. It worked to lean the mixture when you let off the throttle.

Cyberborikusa, Have you tried to see if the firing out the exhaust continues for as long as you are off the throttle?
Or does it eventually stop? Does it do it in each gear all the time?
Try advancing your timing to 10 degrees of advance of its not already set there. This will require resetting your idle speed a bit if you need to make an adjustment to the timing.
And YES it could damage your exhaust system?

Firing back thru the induction system is indicative of a lean condition possibly from a vacuum leak, or early timing.
But this is not what you describe as your problem.

[majicwrench]

I would not change your timing. I would not change your jetting.

The carb setup you took off likely had some sort of decelleration device. All cars had something to keep rpms up a bit and, like Roy said, keep it lean enough on decell. You probably removed/disabled such device.
Yes, and exhaust leak contributes to backfire by letting fresh o2 into exhaust
Sure, bad backfire can damage exhaust system/

Backfire is caused by unburned fuel in hot exhaust system. When you decell throttle slams shut, gets almost no air into cylinder. Vacuum goes way up with throttle shut, sucks like mad from idle port, producing rich mixture. Rich mixture/no air won't burn well in cylinder, so it pumps some unburned fuel into exhaust system. Hot system, some air in system--BANG!

Some things to try...obviously, fix any exhaust leaks. Idle mix set?? Too rich of an idle mix with contribute to problem. Idle speed?? A little on the high side will help improve backfire issues.
Some backfire no big deal, kinda cool on decell anyway. What you don't want is that Big Bang.
Good Luck!

[cyberborikua]

The backfire I suspect is air coming in through the exhaust manifold, I noted smoke coming off the pre-heater plate as the engine is warming up. I will adjust the lose bolt to see how it goes. The backfiring itself is not loud and it occurs mostly in first three gears. The vacuum advance going into the distributor could not be connected for the 34 ADF has no port for it. There could be issues with idling at the carb level, but it cold starts and idles right up and the engine is very steady in low idle once it reaches 190 temp.

[majicwrench]

You need to figure a way to hook up vacuum advance, it is critical to good mileage and smooth running.

[cyberborikua]

I wonder how the vacuum advance can be hooked up. Would lack of it be causing the exhaust backfiring?

[RoyBatty]

It needs to be connected to "ported" vacuum.
NOT regular manifold vacuum.
There are usually plugged ports at the base of the carb that you should be able to open up and install a brass tube to.
If you do this, make sure the port you use goes to a position in the carb throat that is near the edge of the throttle plate when the throttle is partially opened.
Also make sure that the tube you install is full sealed to the base of the carb.
Good Luck.
Also, You indicated initially that "but sometimes it makes a kind of a loud bang".
IF this is the case then YES, you could blast open a muffler.
If it doesn't do this, and you just have a soft popping and burbling out the back with an occasional soft pop, then you are experiencing a normal situation for a non computer controlled car.

[majicwrench]

Many cars have the vacuum advance plumbed into manifold vacuum and they work fine that way. If Fiat uses ported vacuum, I would certainly try to find a ported vacuum source.

[RoyBatty]

Manifold vacuum would pull your distributor to FULL advance all the time.
Ported vacuum only advances the timing at part throttle cruise.
Full advance all the time is not good.

[cyberborikua]

Oh no, not a blast that would attract police attention. All the backfire I would describe it as mild. Take a look at my You tube videos. You may like to copy and paste into browser if unable to click on links.

https://www.youtube.com/watch?v=CD0MRP_ ... su7Lbv3qJQ (high idle after cold start)

https://www.youtube.com/watch?v=iNYpAyQ ... su7Lbv3qJQ

What do you guys think now?

[RoyBatty]

Drive it.

[So Cal Mark]

a rich mixture can cause your popping but also a straight-through muffler will cause the same thing. The lack of vacuum advance is not the problem and you can use manifold vacuum with the advance, though it's usually connected that way if you have performance cams. Especially cams that are too large for ideal street use

[cyberborikua]

a rich mixture can cause your popping but also a straight-through muffler will cause the same thing. The lack of vacuum advance is not the problem and you can use manifold vacuum with the advance, though it's usually connected that way if you have performance cams. Especially cams that are too large for ideal street use

Thanks. So besides fuel economy, what other issues the unplugged vacuum advance cause? I mean, besides the popping, the car seems to run well as seen in the videos. I have no idea how to connect it via the intake vacuum.

[So Cal Mark]

vacuum advance is for fuel mileage. It's not hard to change the mechanical advance and leave the vacuum disconnected